There is a general need in research and enforcement activities relating to environmental pollution, and similar undesirable conditions, for reliable and accurate systems for the monitoring of such conditions. For example, in industrial establishments there is a need for a system which will make available the time-history of the noise levels throughout an entire work shift, without requiring the services of a trained industrial hygienist who would be employed to perform personally an accurate noise level-versus-time survey during said work shift. Furthermore, there is a need for a technique for making measurements of noise, or other adverse conditions, in areas where only the workers have access, and to enable computation of the threshold limit values of the monitored conditions. In the case of noise, it is desirable for administrators to be able to examine a noise environment and to determine just when noise exposure is imposed on a worker and to obtain information as to whether administrative controls are required. The same considerations apply for other environmental conditions, such as exposure to carbon monoxide, hydrogen sulfide, hydrocarbons, halogens, heat stress, or the like, where it is important for researchers to obtain data on a concentration-versus-time basis.
In the case of noise pollution, various noise dosimeters have been devised, for example, those which continuously integrate the sound level throughout a work shift and present a single number at the end of the day which can be related to an equivalent sound pressure level. However, the prior art devices do not preserve the actual sound levels and cannot present them in an actual reconstruction indicating exactly when the noise events occurred. None of the prior art dosimeters now commercially available provides the ability to view the day's noise as it varied with time during the work shift.
Some of the prior systems devised are disclosed in the prior U.S. Pat. Nos. Thus, broadly speaking:
Fletcher, 3,868,856 measures sound pressure level using a transducer, produces a signal proportional thereto, and records it on magnetic tape.
Labarber et al., 3,828,279 shows a system using transducer and time modulator, with a gate controlling strings of pulses passing to a pulse counter and memory circuit which provides digital representation and storage of condition inputs to transducers.
Quinn, 3,892,133 uses audio level sensor which provides a sensor signal indicating the amplitude. A recorder is used to indicate the level of sound over a period of time.
Rackey et al., 2,590,460 shows sound level and duration measurement using pulses and means for counting the pulses.
Von Wittern et al., 2,884,085 provides noise exposure meter indicating total time that noise exceeds a predetermined level.
Peake et al. 3,802,535 employs a system wherein pulse frequency is proportional to sound intensity variation, with a binary pulse counter.
Church et al., 3,236,327 uses time recording device triggered by sound level sensing system, and employs electrochemical recording means.
However, as pointed out above, the prior art does not accomplish the desired objectives.